This invention relates to distillation methods and apparatus in which volatile components are stripped from a liquid by a stripping gas at an elevated temperature, and more particularly to the steam distillation or stripping of ammonia from coke plant liquors, commonly known as weak ammonia liquor, or WAL. While the specific discussion and examples cited herein refer to the treatment of weak ammonia liquor, or WAL, and it is in connection with the treatment of such liquor that the invention has been found most useful, the invention is equally applicable to any process which treats a liquid containing one or more volatile components with steam or other hot primary stripping gas in order to remove such components from the liquid and provide a gaseous effluent which is rich in volatile components and a liquid effluent which is poor in such components.
In the thermal decomposition of coal to produce coke for use in the steel industry, vapors are given off which contain various components including tars, light oils, phenols, naphthalenes, hydrogen cyanide, hydrogen sulfide, carbon dioxide, ammonia and moisture. Various methods for the isolation of these components have been devised, some of which are described in the U.S. Steel publication "The Making, Shaping and Treating of Steel" 9th ed., Harold E. McGannon, Ed. (1971) pp 165 et seq.
The vapors given off during thermal decomposition, or coking, of coal are customarily initially cooled in a water spray tower known as a primary cooler. During such cooling, the excess moisture in the vapor condenses and absorbs ammonia, ammonium compounds and other lesser contaminants and is then known as weak ammonia liquor. The weak ammonia liquor may be recirculated through the primary cooler several times until the liquor becomes saturated with ammonia and other components and must be treated to remove the ammonia. As is described in the cited U.S. Steel treatise, the most commonly used process for ammonia recovery from weak ammonia liquor is the semi-direct process. In this process, weak ammonia liquor, which normally contains approximately 5000-9000 p.p.m. of ammonia, is subjected to a three leg distillation process. This three leg process is necessitated by the fact that ammonia is present in the weak ammonia liquor in two broad physically distinct types of combinations, known as "free" ammonia and "fixed" ammonia. The term "free" ammonia encompasses dissolved ammonium salts of carbonate, sulfide and cyanide ions, solutions of which are thermally decomposable into their gaseous components, i.e. ammonia and acid. The term "fixed" ammonia encompasses dissolved ammonium salts, such as the chloride, thiocyanate and sulphate, solutions of which are not thermally decomposable, but which require the addition of an alkaline reagent which will chemically react with the ammonium ion, converting it to strippable ammonia gas.
The weak ammonia liquor is first subjected to a steam distillation in a so-called "free leg" of a still to drive off the "free" ammonia. The remaining liquor, containing the fixed ammonia, exits from the bottom of the "free leg" and passes into a mixing chamber where it is intimately mixed with an aqueous slurry of calcium hydroxide, commonly known as milk of lime. The resulting weak ammonia liquor/calcium hyroxide mixture from the so-called "lime leg" is then passed into the "fixed leg" of the still where ammonia is stripped from the liquor by the use of steam. The hot effluent liquors, or still bottoms, containing only a trace amount of ammonia, are passed out the bottom of the still and into a waste liquor sump from which they may be removed for further treatment before disposal.
Both the free leg and the fixed leg of a conventional still are vertically upright cylindrical columns, usually positioned one upon the other, having a number of horizontal trays spaced along the height of each column. Each of these trays is equipped with bubble cap assemblies or sieve-type orifices through which steam may pass upwardly from tray to tray and is usually also equipped with downcomers through which weak ammonia liquor may descend from tray to tray, all as well known to those skilled in the art. Ascending steam is intimately mixed with descending weak ammonia liquor at each tray level.
In a conventional ammonia still, steam is supplied to the bottom of the fixed leg at a point below the lowest tray, but above the level of the still bottoms, under pressure sufficient to force the steam upward through the tray openings of both the fixed leg and the free leg to the top of the still column, where the steam leaves the free leg carrying with it the ammonia and other volatile gases. The steam/ammonia stream is then passed through a dephlegmator to cool and condense a portion of the steam, thereby enriching the ammonia content of the gas stream. Following the dephlegmator, the enriched steam/ammonia stream may either be passed to a sulfuric acid saturator, in which ammonium sulfate is produced, or passed to an incinerator for destruction of the ammonia and other combustible gases.
A second type of ammonia still substitutes a solution of sodium hydroxide for the conventional milk of lime slurry in order to avoid certain operating problems which occur in conventional stills. This more recent still is described in an article in the March 1975 issue of I&SM entitled "A New Method of Treating Coke Plant Waste Waters" by A. C. Naso and J. W. Schroeder, beginning at page 27.
Both the conventional and the more recently developed ammonia stills utilize steam, or occasionally some other hot gas, as a direct contact stripping medium. Steam has historically been attractive for such use both because of its low cost and its ready availability in steel plants. Recently, however, the cost of steam production has increased drastically due primarily to increased fuel cost. For this reason there is a demand for new and more efficient distillation systems.
Attempts have been made in the past to increase both the thermal efficiency of steam distillation systems in general and weak ammonia liquor (WAL) distillation systems in particular. For example, several inventors have attempted to use the waste heat from the steam and ammonia vapor passing from the top of the ammonia still to heat the incoming WAL. Several such arrangements have been patented. Prior inventors have also attempted to take advantage of the higher temperature of the still bottom effluent to initially heat the incoming WAL. In these arrangements the WAL entering the still has been passed first through a heat exchanger in heat exchange relationship with the hot still bottoms. Several such arrangements have also appeared in U.S. patents. While heat values would seem to be conserved by this sort of arrangement, the efficiency of the distilling operation is not greatly increased. This is because it is desirable from an efficiency standpoint to make maximum use of the heat in the incoming steam within the distilling column itself. Consequently the vapors leaving the column should be as cool as possible. By preheating the incoming WAL with heat from the distillation operation itself and then passing such preheated WAL into the top of the still, the upper portions of the still are heated with the result that the vapors entering the condenser and/or leaving the top of the still tend to be hotter rather than cooler. As a result more, rather than less, heat is exhausted from the distillation apparatus with a consequent loss in thermal efficiency.
It has also been suggested in the past to use other stripping mediums than steam in an attempt to increase the efficiency of an ammonia distillation system. Thus, in a distillation arrangement shown in U.S. Pat. No. 1,244,903 to Schuster, tar-free superheated coke oven gas is used as a substitute for steam in an ammonia still. This hot gas serves both as the heat source for the still and as the carrier medium to strip ammonia from ammonia containing liquor. Increased efficiency is asserted, but since the coke oven gas must be heated from some external source of heat, just as steam must be, the thermal efficiency is not greatly increased. The previously noted article by Naso and Schroeder also discloses the use of heated air to replace all or a part of the usual steam for stripping. The use of steam can be essentially eliminated in such an arrangement except to heat and humidify the air. However, any blend of steam and air can also be used to strip the ammonia from the WAL. The thermal efficiency of such arrangements is again not greatly increased, however, because an external heat source is used to preheat the air.